Thermal ink-jet printers operate by using a resistance element that is controllably energized to expel ink droplets through a nozzle onto a print medium. Each heater resistor and its associated nozzle is located in a firing chamber, into which ink is introduced from an ink refill slot via an ink feed channel. There are typically a plurality of heater resistors and associated nozzles in a given printhead, permitting the printing of alphanumeric characters, area-fill, and the like.
In previous Hewlett-Packard color ink-jet printers having a resolution of 180 dots-per-inch, satisfactory printing was obtained using the same nozzle diameters for the color inks and for the black ink.
However, in a higher resolution color ink-jet printer, it is desirable to have a larger drop mass for the black cartridge than for the CYM cartridges. This is because the black dots on paper are made from a single color and must be made larger to accommodate this fact as well as achieve optimal text print quality, which requires larger drop mass. Since red, green, and blue are made from two drops (see the Table below), the resultant dot size on the print medium is larger than for cyan, yellow, or magenta alone.
Table ______________________________________ Printing Color in a CYMK Printing System Cartridge Colors # of Desired Color Cyan Yellow Magenta Black Drops ______________________________________ Cyan X 1 Yellow X 1 Magenta X 1 Red X X 2 Green X X 2 Blue X X 2 Black X 1 ______________________________________
If the same larger drop mass from the black cartridge is used for the cyan, yellow, and magenta cartridges, the resultant red, green, and blue dot size would be unacceptably large. By designing a lower drop mass cartridge for the cyan, yellow, and magenta colors, optimal dot size is achieved for all colors (C,Y,M,R,G,B,K).
Furthermore, in a heated printing system, the drop mass of all cartridges will increase as the cartridge heats up from being exposed to the heated printing environment.
Prior solutions to the problem of droplet size have been accomplished by using totally different architectures for the color and black cartridges. For example, U.S. Pat. No. 4,746,935, issued to Ross R. Allen and assigned to the same assignee as the present application, teaches that in order to change the droplet size, the size of the resistor, the nozzle, the firing chamber, and the ink feed channel all must be changed. Smaller size droplets are created by reducing all four elements relative to those for a larger size droplet.
There remains a need to provide a pre-determined droplet size, yet keep the pen architecture as simple as possible.